1) Field of the Invention
The present invention relates to a holographic recording medium and a method of manufacturing the holographic recording medium.
2) Description of the Related Art
Recently, collinear holographic recording systems in reflection geometry have been proposed to solve disadvantages in the angular multiplexing in the transmission geometry (See, for example, Japanese Patent Application Laid-open No. 2002-123949). An optical recording medium used for such systems includes a plastic substrate provided at least below a recording layer, and a reflecting layer on its rear surface. A recording beam and a reference beam are passed through a coaxial optical path and incident on the recording layer of the optical recording medium. An interference pattern is formed on this recording layer by interference of the recording beam incident on the recording layer with the reference beam transmitted by the substrate, reflected by the reflecting layer, and transmitted again by the plastic substrate below the recording layer. At the same time, an interference pattern is formed thereon by the interference of the reference beam incident on the recording layer with the recording beam transmitted by the recording layer and the plastic substrate below the recording layer, reflected by the reflecting layer, and transmitted again by the plastic substrate below the recording layer. Therefore, if the plastic substrate shows birefringent, rotary polarization occurs to each of the recording beam and the reference beam after each light is transmitted by the plastic substrate. As a result, a mismatching occurs between a plane of polarization of the recording beam and that of the reference beam, thereby disadvantageously reducing amplitudes of the interference patterns. The amplitude reduction of the interference patterns causes a reduction in a signal-to-noise ratio (SN ratio, a ratio of an intensity of the signal light to a noise) in recording.
Meanwhile, development of a low birefringent plastic substrate to be used in a currently available optical disk is underway. However, a birefringence of the low birefringent plastic is not low enough. Therefore, even if the low birefringent plastic substrate is used as the substrate of the holographic recording medium, the mismatching occurs again between the plane of polarization of the recording beam and that of the reference beam. As a result, the recorded light cannot be often reproduced and the SN ratio is reduced.